1. Field of Invention
The present invention relates to an electronic timepiece that adjusts the time based on signals received from positioning information satellites such as GPS satellites.
2. Description of Related Art
The Global Positioning System (GPS) in which satellites (GPS satellites) orbiting the Earth on known orbits transmit signals carrying superposed time information and orbit information, and terrestrial receivers (GPS receivers) receive these signals to determine the location of the receiver, is widely known. The GPS receiver captures signals from a plurality of GPS satellites, acquires satellite information including accurate time information (GPS time information) and orbit information from each of the captured satellites, performs a positioning (navigation) calculation using the acquired satellite information, and thereby acquires its own location. The time required to acquire the satellite information therefore depends upon the strength of the satellite signal, and the positioning calculation can take a long time to conclude.
Japanese Unexamined Patent Appl. Pub. JP-A-2003-344523 teaches a GPS receiver that stores previously acquired satellite information and positioning information in backup memory, and has a hot start mode and a cold start mode. The hot start mode uses the information stored in backup memory to predict the GPS satellites that currently can be acquired, and shortens the time required from startup to determining the position by attempting to capture those satellites. The cold start mode attempts to sequentially capture all GPS satellites in the constellation. When this GPS receiver starts in the cold start mode, the timeout time is set longer than when the receiver starts in the hot start mode to improve the probability that the positioning calculation converges.
Electronic timepieces that acquire accurate time information and orbit information from a GPS satellite and adjust the current time kept by the timepiece to the accurate local time have also been developed as a type of GPS receiver. However, a problem with battery-powered timepieces such as a wristwatch is that battery power can drop sharply if a long time is required for the positioning calculation, causing the system to shutdown. Therefore, a problem that is presented particularly by battery-powered timepieces is optimizing power consumption by setting the timeout period of the positioning calculation according to the reception conditions. Because the method taught in JP-A-2003-344523 sets a longer timeout period in the cold start mode, the positioning calculation can take a long time in the cold start mode if the signal strength is weak.